In 1966, Mr. Strong left his position as President of the Power Corporation of Canada to head Canada’s External Aid Office, later reorganized as the Canadian International Development Agency. From November 1970 until December 1972, Mr. Strong was Secretary-General of the United Nations Conference on the Human Environment. He then became the first Executive Director of the United Nations Environment Programme. Mr. Strong returned to Canada as President, Chairman and Chief Executive Officer of Petro-Canada (1976-78). He served as Chairman of the International Energy Development Corporation from 1980 to 1983 and of the Canada Development Investment Corporation from 1982 to 1984.

Since 1985, he has been an Under-Secretary-General of the United Nations and has held a variety of senior posts, including Secretary-General of the 1992 United Nations Conference on Environment and Development.